1. Field of the Invention
The present invention relates to a dual type vehicle power-supply apparatus including a high power-supply voltage supply system and a low power-supply voltage supply system.
2. Description of Related Art
A dual type vehicle power-supply apparatus including two batteries providing different power supply voltages is used in a hybrid vehicle, for example. The high power-supply voltage supply system of such a dual type vehicle power-supply apparatus is intended to supply electric power to large electrical loads which are turned on and off at frequent intervals. By supplying electric power to such large electrical loads in high voltage, advantages of reduction of size and weight of a power feeding system and electrical loads, reduction of power transmission losses, and increase of the cycle life of the power supply apparatus can be obtained. The low power-supply voltage supply system is intended to supply electric power to electrical loads which require relatively small power in low voltage, such as electronic devices. Generally, a lead-acid vehicle battery, which is low in price and easy to replace, is used for the low power-supply voltage supply system.
It is common that the dual type vehicle power-supply apparatus is configured to generate a high voltage for its high power-supply voltage supply system, and produces a low voltage for its low power-supply voltage supply system by stepping down the generated high voltage by use of a DC-DC converter, for example.
In a hybrid vehicle, the high power-supply voltage supply system must supply a large amount of electric power for starting a vehicle engine very often. Accordingly, there is known a technique for transmitting electric power back to the high power-supply voltage supply system from the low voltage power-supply supply system to thereby increase the feeding capacity of the high power-supply voltage supply system before starting the engine, as disclosed in Japanese Patent Application Laid-open No. 2002-176704 filed by the same inventors of the present application. Such a technique makes it possible to downsize the high power-supply voltage supply system.
However, although the backward power transmission technique described above makes it possible to use a high power-supply voltage supply system having a smaller feeding capacity (smaller discharging capacity), it invites a problem that the lead-acid battery of the low power-supply voltage supply system, which is low in price and has a relatively short cycle life, reaches the end of its useful life too soon. In addition, the backward power transmission technique has another problem in that the fuel consumption of a vehicle is lowered by the power transmission loss produced during the backward power transmission.